Transport refrigeration systems condition the air of a refrigerated space of a vehicle, hereinafter called the "conditioned space", such as a straight truck, the trailer of a tractor-trailer combination, a refrigerated container, and the like, with the air of the conditioned space being controlled to a narrow temperature range around or adjacent to a predetermined set point temperature. Such systems conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. The mechanical refrigeration cycle requires a refrigerant compressor driven by a prime mover, which often includes a dedicated internal combustion engine, such as a diesel engine. Because of the suspected depleting effect of CFC's on stratospheric ozone (O.sub.3), and the detrimental effects of diesel emissions, practical alternatives to the use of CFC's are being sought.
The use of a cryogen, i.e., a gas which has been compressed to a very cold liquid state, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2), in a transport refrigeration system is particularly attractive because, in addition to eliminating the need for a CFC, it also eliminates the need to transport a compressor and associated prime mover. Refrigeration systems of which we are aware which utilize a cryogen, implement a cooling cycle by circulating the cryogen through a fluid flow path, or flow paths, of a heat exchanger which is in heat exchange relation with air from the conditioned space. When a heating cycle is required to hold the set point temperature, or a defrost cycle is required, the cryogen is heated by a suitable burner and combustible fuel, and the heated cryogen is circulated through the fluid flow path, or flow paths, of the conditioned space heat exchanger. Thus, cryogen is expended during a cooling cycle, and cryogen plus a fuel associated with the heat source, such as propane, liquid natural gas, and the like, are both expended to implement heating and defrost cycles.
It would be desirable, and it is an object of the present invention, to provide new and improved cryogenic based air conditioning and refrigeration systems, suitable for both transport and stationary applications, which more effectively and efficiently utilize the cryogen, for lower cost operation, as well as for an extended operating time in a transport application for a given on-board vessel of cryogen. The term "refrigeration system" will be hereinafter used to generically cover both air conditioning and refrigeration applications, transport and stationary.
Cryogens in a liquid state may be delivered at much colder temperatures than temperatures associated with fresh loads, such as lettuce, for example, and/or the "as delivered" temperature may not be cold enough for certain frozen loads. Thus, it would be desirable, and it is another object of the invention, to provide a refrigeration system utilizing a cryogen which effectively conditions a space containing a fresh load, while minimizing the chance for spot freezing, without compromising the ability of the system to adequately condition a frozen load.